In order to support the communication requirements of high-speed data transmission applications of e.g. 25, 40 or 100 Gbps, optical links are used when links via an electrical wire have a too low bandwidth. When using such an optical link for transmitting a signal from a first electronic component to a second electronic component, the electrical signal to be transmitted is first converted into an optical signal, then the optical signal is coupled into an optical fiber via an optical transmitter and transmitted to the second electronic component via the optical fiber. At the second electronic component, the optical signal is received by means of an optical receiver and converted back into an electrical signal. This converted electrical signal is further processed in the second electronic component.
Optoelectronic components that perform the transduction between the optical and electrical signals are often referred to as transceivers, E/O engines or EOE engines.
As shown in FIG. 1, such a transceiver 100 which is used for converting an electric signal into an optical signal and vice versa, includes an electrically insulating substrate 102, for instance a printed circuit board (PCB) or a flexible printed circuit (FPC). The transceiver 100 further comprises a plurality of signal input lines 104 which are arranged as differential signal pairs and end in a front end contact region for electrically contacting further electronic components, for instance via another printed circuit board.
The other peripheral end of the signal input lines being opposed to the front end contact region 106 is connected to an electronic transmission unit 108 in a circuit connecting region 110. The electronic transmission unit 108 comprises driver circuitry for driving optical senders, for instance an array of vertical cavity surface emitting lasers (VCSEL) 112. The optical signal emitted by the laser diode array 112 is internally coupled to an optical conductor, for instance an optical fiber.
Furthermore, the transceiver unit 100 comprises a photo detector array 114, which comprises for instance photodiodes, such as so-called PIN diodes (p-intrinsic-n photodiodes). These PIN diodes are coupled to the optical fiber for receiving an optical signal and converting same into an electrical signal. The output of the PIN diodes 114 is coupled to an amplifier unit 116, which may comprise an array of transimpedance amplifiers (TIA) connected to respective outputs of the array of photodiodes 114.
A plurality of electrical signal output lines 120 are provided for connecting the front end contact region to the output terminals of the amplifier circuit 116. The signal output lines 120 are formed as differential lines analogously to the signal input lines 104.
A ground plane layer 118 is provided within the substrate 102 with a well-defined distance towards the input and output signal lines 104 and 120, respectively.
The laser diodes 112 and the driver circuit 108 as well as the PIN diodes 114 and the belonging amplifier unit 116 are all placed on the substrate 102 in a way that they are surrounded by the ground plane layer 118. As proposed in the international application PCT/EP2013/063694, the ground plane layer 118 is provided with openings 124 in the region of the front end contact region 106 in order to improve the signal quality at the transition point from the E/O engine 100 to e.g. a further printed circuit board (not shown in the figure).
The inventors of the present invention found that for the design shown in FIG. 1, due to parasitic effects of the bond pads and ESD protection devices on the chip, the impedances of the front end of the transmission unit 108 as well as the transimpedance amplifier unit 116 exhibit a capacitive nature. Therefore, significant impedance drops in the area of the circuit connection region 110 of the electronic transmission unit 108 and of the amplifier unit 116 are observed. This impairs the performance of the optoelectronic unit and makes it difficult to meet the return loss specification.
On the other hand, ideally, the interconnection system has to carry signals without distortion. One type of distortion is called crosstalk. Crosstalk occurs when one signal creates an unwanted signal on another signal line. Generally, crosstalk is caused by electromagnetic coupling between signal lines and is therefore a particular problem for high-speed, high-density interconnection systems. Electromagnetic coupling increases when signal lines are closer together or when the signals they carry are of a higher frequency. Both of these conditions are present in a high-speed, high-density interconnection system.
FIG. 2 shows the result of a time-domain reflectometry (TDR) measurement at the circuit connecting region 110 obtained with 20 ps (20 to 80%) pulses without any impedance matching measures. It can be seen from FIG. 2 that a significant pulse is reflected back indicating an undesired disturbance in this region.
Consequently, there exists the need of providing an interconnect structure and a respective optoelectronic module that has an improved signal integrity and a reduced crosstalk, at the same time maintaining a small form factor and a cost-effective construction.
This object is solved by the subject matter of the independent claims. Advantageous developments of the inventive interconnect structure and optical module are the subject matter of the dependent claims.
The invention is based on the finding that the impedance can be increased to an acceptable level in the region of the circuit connecting terminals by introducing clearances into the ground plane layer of the E/O engine substrate. Advantageously, this can be done below the terminals of the electronic transmission unit as well as below the terminals of the amplifier unit. By providing such a ground clearance configuration, crosstalk (XT), mode conversion and common mode return loss (CM RL) are not significantly compromised, whereas the impedance can be matched in order to meet the requirements regarding return losses.
In particular, the clearances are each allocated to one pair of differential signal leads and are separated from one or two neighboring clearances by a ground plane web of exactly defined dimensions. In order to avoid crosstalk, each of the clearances is separated from the next clearance by a web that has a width of at least 30 μm
In order to ensure that a uniform impedance matching is established for each of the signal lead pairs, the clearances have a contour that matches the contour of the respectively belonging signal leads.
When analyzing different options for the particular dimensions and the layout of the signal leads, it surprisingly turned out that a ratio between the distance of each lead of one pair towards the adjacent outer boundary of the clearance, and the distance between both leads should be approximately ½ in order to reach a sufficiently high impedance. For instance, each lead can be distanced 50 μm from the respective adjacent outer boundary of the clearance and 100 μm from the other lead of one pair.
Further, it could be shown that length of the clearance of at least 700 μm is advantageous for reaching sufficiently high impedance.
The present invention can most effectively be used with an optoelectronic module comprising at least one electronic unit for outputting and/or receiving electric signals and at least one optical unit for converting the electric signals into optical signals and/or vice versa.
In the context of the present invention, an optoelectronic module refers in general to a system comprising optoelectronic components for transmitting or receiving an optical signal connected to a driver and/or receiver electronics. Optoelectronic components in the present context are devices arranged to convert electrical energy into optical energy or optical energy into electrical energy, i.e. light sources and photo detectors, such as laser diodes and photo diodes, as mentioned above. Often, the laser diodes are vertical cavity surface emitting lasers (VCSEL) and as photo diodes p-intrinsic-n photo diodes may be used.
Typically, such a module will also comprise an interface allowing the module to be connected to one or more optical fibers as well as control electronics to adjust the operating parameters of the optoelectronic components. For example, the operation of a laser diode typically requires an adjustable bias current, modulations current and optionally a pre-emphasize. Often, such modules will support more than one channel, such as two, four, eight, twelve or sixteen channels, but any number of channels is conceivable depending on the application. For such a use, the light sources and photo detectors are often available in arrays, such as 1×N arrays or 2×N arrays, wherein N is a positive integer. Strictly, a 2×N array is referred to as a matrix, but in order to simplify notation, only the term “array” is used in the following. Furthermore, the embodiments shown in the present application always use a 4-channel arrangement in line with the application as a quad small form-factor pluggable (QSFP) E/O engine, but the invention is of course not limited to such an arrangement.
In order to convert an electrical data signal into a signal suitable for driving a light source to emit an optical signal comprising this data signal, a driver circuit is required. Similarly, a receiver circuit is required to convert received optical signals into an electrical signal suitable for further transmission in the system. Such driver and receiver circuits are well-known in the art and they are typically provided as integrated circuits either as transmitter chips (comprising driver circuits), receiver chips (comprising receiver circuits) or transceiver chips (comprising a driver and receiver circuit).
A receiver chip is often also referred to as a TIA chip (transimpedance amplifier chip) or an LIA chip (limiting impedance amplifier chip). These chips comprise data pins/pads for receiving/transmitting the electrical data signals to/from a host system and connecting pads for connecting to the optical devices, i.e. connecting pins/pads for connecting to the optical side of the chip, i.e. light sources or photo detectors.
The advantageous characteristics that are reached by the present invention can be exploited to a particularly high extent when using the inventive interconnect structure in an active optical cable (AOC) assembly that inputs and outputs electrical signals but conducts same by means of an optical conductor. The active optical cable assembly may be either a direct point-to-point connection or may also be structured as a fan out cable, meaning one input and a plurality of outputs. The active optical cable technology improves speed and distance performance of the cable without sacrificing compatibility with standard electrical interfaces.